U.S. patent application number 11/714135 was filed with the patent office on 2007-10-04 for wire electric discharge machine.
This patent application is currently assigned to FANUC LTD. Invention is credited to Yuki Kita, Shinji Okuda, Hikaru Yamane.
Application Number | 20070228015 11/714135 |
Document ID | / |
Family ID | 38268721 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070228015 |
Kind Code |
A1 |
Kita; Yuki ; et al. |
October 4, 2007 |
Wire electric discharge machine
Abstract
A lower arm is passed through a slot in a machining tank. The
slot of the machining tank is provided with a seal section for
sealing with a plate-shaped seal member. The movable table is moved
by a servomotor. The machining tank and a workpiece are fixed on
the movable table. At the seal section, a seal member on the
machining tank side slides with respect to the plate-shaped seal
member, thereby generating a sliding resistance. This sliding
resistance increases as a sludge produced by electric discharge
adheres to the seal section, thereby distorting the lower arm and
influencing the machining accuracy. Accordingly, a load that acts
on the servomotor is detected, and a sliding resistance load at the
seal section is obtained. When a reference load is exceeded by the
detected resistance load, arrival of the seal section at the timing
for maintenance is displayed and notified.
Inventors: |
Kita; Yuki;
(Minamitsuru-gun, JP) ; Okuda; Shinji;
(Minamitsuru-gun, JP) ; Yamane; Hikaru;
(Minamitsuru-gun, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700, 1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
FANUC LTD
Yamanashi
JP
|
Family ID: |
38268721 |
Appl. No.: |
11/714135 |
Filed: |
March 6, 2007 |
Current U.S.
Class: |
219/69.12 ;
219/69.19 |
Current CPC
Class: |
B23H 7/02 20130101 |
Class at
Publication: |
219/69.12 ;
219/69.19 |
International
Class: |
B23H 7/02 20060101
B23H007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2006 |
JP |
103548/2006 |
Claims
1. A wire electric discharge machine, in which a lower arm
penetrates a slot formed in one side plate of a machining tank
movable in an axial direction of the lower arm and a direction
perpendicular to the axial direction, and which has a seal section
configured to prevent a machining fluid from leaking out through
the slot of the machining tank, the wire electric discharge machine
comprising: load detecting means for detecting a sliding resistance
load at the seal section; determination means for determining
whether or not a predetermined load is exceeded by the load
detected by the load detecting means; and warning means which
issues a warning to prompt maintenance of the seal section when the
determination means concludes that the predetermined load is
exceeded by the detected load.
2. The wire electric discharge machine according to claim 1,
wherein the warning means is a display unit, a signal lamp, or an
alarm generator configured to generate an alarm sound.
3. The wire electric discharge machine according to claim 1,
wherein the load detecting means detects a load on a motor for
driving the movable table.
4. The wire electric discharge machine according to claim 1,
wherein the seal section is composed of a machining-tank-side seal
member mounted on the side plate of the machining tank so as to
surround the slot and a plate-shaped seal member which presses the
first seal member against the side plate of the machining tank,
thereby sealing the slot, and the load detecting means is a current
detector which detects a drive current of a motor for driving the
movable table with respect to the lower arm in a longitudinal
direction of the slot of the machining tank, perpendicular to the
axial direction of the lower arm, thereby detecting a sliding
resistance generated when the machining-tank-side seal member
slides with respect to the plate-shaped seal member as the movable
table is driven to move by the motor.
5. The wire electric discharge machine according to claim 1,
wherein the load detecting means is composed of a disturbance
estimation observer for estimating a load on a drive motor for
driving the movable table.
6. The wire electric discharge machine according to claim 1 or 2,
wherein the load detecting means is a load cell attached to a
member which is influenced by the sliding resistance load at the
seal section.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wire electric discharge
machine, and more particularly, to maintenance of a seal section of
a machining tank.
[0003] 2. Description of the Related Art
[0004] In a wire electric discharge machine, a dipping process is
generally performed such that a workpiece is machined in a
machining fluid in order to improve the machining speed and
accuracy.
[0005] The dipping process requires a machining tank to hold the
machining fluid (water) in which the workpiece is dipped. Usually,
upper and lower wire guides that support a wire electrode are
located above and below the workpiece, respectively, and the
electrode is stretched between the wire guides. The wire electrode
is run as voltage is applied between the electrode and the
workpiece, and the workpiece is moved relatively to the electrode
as it is subjected to electric discharge machining.
[0006] An upper arm and a lower arm that are provided individually
with the wire guides are located above and below the workpiece,
respectively, and a drive mechanism is provided for moving the
workpiece and the machining tank in two directions (X-- and Y-axis
directions) perpendicular to the stretching direction (Z-axis
direction) of the wire electrode. The distal end (on which the
lower guide is disposed) of the lower arm that is provided with the
lower wire guide for guiding the wire electrode must be situated in
the machining tank, and the lower arm can move relatively to the
tank. Therefore, one side plate of the machining tank is formed
with a slot, through which the lower arm is passed, and provided
with sealing means for preventing the machining fluid from leaking
out through the slot.
[0007] In general, the sealing means is composed of a
machining-tank-side seal member of a low-friction material and a
plate-shaped seal member, which are opposed to each other. The
tank-side seal member is located around the slot of the machining
tank, while the plate-shaped seal member covers the slot. These two
seal members can prevent leakage of the machining fluid when they
are brought into contact with each other. However, sliding friction
is generated as the machining tank and the lower arm move
relatively to each other. This sliding friction impedes the
positioning accuracy of the workpiece, which moves together with
the tank, with respect to the wire electrode, thereby reducing the
machining accuracy.
[0008] There is a known method (see Japanese Patent Application
Laid-Open No. 7-132418) for stopping the leakage of the machining
fluid that is attributable to an error in thickness or warp of the
sealing means, thereby preventing reduction of the machining
accuracy. According to this method, seal retaining means is
provided for pressing the seal members against the machining tank
under controlled pressure. A load on a movable table drive
mechanism for driving a movable table, which is mounted with the
workpiece and the machining tank, is detected, and the retaining
force of the seal retaining means that presses the seal section
against the machining tank is controlled based on the detected
load. Alternatively, an amount of deformation of the lower arm that
projects into the tank is detected, and the retaining force of the
seal retaining means is controlled in accordance with the detected
deformation amount.
[0009] If the electric discharge machining is carried out in the
machining fluid, moreover, sludge with particle diameters of
several micrometers scatters from the electrode wire and the
workpiece and floats in the water. If the sludge gets into a narrow
gap between the opposed seal members in contact with each other,
therefore, the frictional resistance increases and exerts a bad
influence on the positioning accuracy.
[0010] A method (see Japanese Patent Application Laid-Open No.
4-159023 and International Publication WO 01/062423) is known in
which a nozzle for jetting the machining fluid is located in a gap
in the sliding position of the seal section, whereby leakage of the
machining fluid through the seal section is suppressed, and the
sludge is prevented from getting into the seal members.
[0011] If the sludge gets into the seal section and adheres to the
seal members, the sliding resistance increases, and the lower arm
that supports the lower wire guide is distorted. Thus, the
positioning accuracy of the machine is reduced, so that the
machining accuracy worsens. Accordingly, means is provided for
preventing the sludge from getting into the seal section, as
described in Japanese Patent Application Laid-Open No. 4-159023 and
International Publication WO 01/062423. Technically, however, it is
very difficult to remove all the sludge that adheres to the seal
members, and the cost of the wire-cut electric discharge machine is
increased. Further, prolonged use never fails to increase the
sliding resistance load and reduce the positioning accuracy.
[0012] Thus, it is ideal to clean and maintain the seal section
frequently, in order to remove the sludge that adheres to the seal
members. Actually, however, the construction of the seal section is
so complicated that adjustment after the cleaning is difficult. In
consequence, frequent maintenance is a heavy burden for ordinary
users.
SUMMARY OF THE INVENTION
[0013] The present invention relates to a wire electric discharge
machine, in which a lower arm penetrates a slot formed in one side
plate of a machining tank movable in an axial direction of the
lower arm and a direction perpendicular to the axial direction, and
which has a seal section configured to prevent a machining fluid
from leaking out through the slot of the machining tank. The
electric discharge machine comprises load detecting means for
detecting a sliding resistance load at the seal section,
determination means for determining whether or not a predetermined
load is exceeded by the load detected by the load detecting means,
and warning means which issues a warning to prompt maintenance of
the seal section when the determination means concludes that the
predetermined load is exceeded by the detected load.
[0014] The warning means may be a display unit, a signal lamp, or
an alarm generator configured to generate an alarm sound.
[0015] The load detecting means may be configured to detect a load
on a motor for driving the movable table.
[0016] The load detecting means may be composed of a disturbance
estimation observer for estimating a load on a drive motor for
driving the movable table.
[0017] The load detecting means may be a load cell attached to a
member which is influenced by the sliding resistance load at the
seal section.
[0018] In the electric discharge machine according to the present
invention constructed in this manner, the sliding resistance load
at the seal section is detected, and the timing for maintenance of
the seal section is notified automatically. Thus, the maintenance
can be performed to lighten the sliding resistance load at the seal
section before the positioning accuracy of the machine and the
machining accuracy are reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above and other objects and features of the present
invention will be obvious from the ensuing description of
embodiments with reference to the accompanying drawings, in
which:
[0020] FIG. 1 shows a drive section of one embodiment of a wire
electric discharge machine according to the present invention and a
seal section of a machining tank;
[0021] FIG. 2 is a schematic block diagram mainly showing a
controller of the wire electric discharge machine according to the
one embodiment of the invention; and
[0022] FIG. 3 is a flowchart showing maintenance timing detection
notification processing executed by a processor of the controller
of FIG. 2 for each predetermined period during the operation of the
wire electric discharge machine of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] FIG. 1 shows a drive section of a wire electric discharge
machine according to the present invention and a seal section of a
machining tank, which are constructed in the same manner as those
of a conventional wire electric discharge machine.
[0024] A machining tank 2 is fixed on a movable table 1. The tank 2
contains a workpiece cradle (not shown) on which a workpiece can be
mounted and a jig (not shown). The workpiece is fixed on the
workpiece cradle by the jig and is dipped in a machining fluid in
the machining tank when it is machined. The movable table 1 is
driven with respect to a lower arm 3 in the axial direction of the
arm 3 and in a direction perpendicular to the axial direction and
moves on a horizontal plane.
[0025] FIG. 1 shows only drive means (servomotor 4) that drives the
movable table 1 in the direction of an X-axis that is perpendicular
to the axial direction (Y-axis direction) of the lower arm 3. A
ball screw 5 is coupled to an output shaft of the servomotor 4. The
screw 5 is in mesh with a ball nut that is attached to the movable
table 1. Thus, if the servomotor 4 is driven to rotate, its rotary
motion is converted into a linear motion as the ball screw 5 and
the ball nut are coupled together. Thereupon, the movable table 1
moves in the X-axis direction.
[0026] In FIG. 1, reference numeral 6 denotes a guide mechanism
that guides the movable table 1 in movement in the X-axis
direction.
[0027] Although not shown in FIG. 1, a drive mechanism (a
servomotor and combination of ball screw and ball nut) is provided
for moving the movable table 1 and the machining tank 2 thereon in
the axial direction (Y-axis direction) of the lower arm 3.
[0028] A slot that extends long in the X-axis direction is formed
in one side plate of the machining tank 2, and the lower arm 3
penetrates the slot. A lower wire guide (not shown) for guiding a
wire electrode is provided on the distal end of the lower arm 3. A
seal section 7 is located in a position opposite the slot in the
machining tank 2 lest the machining fluid leak through the slot.
The seal section 7 is composed of a seal member (on the machining
tank side) mounted on the side plate of the machining tank 2 so as
to surround the slot, a plate-shaped seal member 7a that presses
the tank-side seal member against the one side plate of the
machining tank 2, thereby sealing the slot, and the like. Further,
another seal member seals a space between the plate-shaped seal
member 7a and the lower arm 3 that penetrates the slot in the
machining tank 2 and a circular hole in the seal member 7a.
[0029] When the servomotor 4 is driven, the movable table 1 and the
machining tank 2 move in the X-axis direction with respect to the
lower arm 3 and the plate-shaped seal member 7a. When the machining
tank 2 moves relatively to the seal member 7a, the seal member of
the seal section 7 that is attached to the side plate of the
machining tank 2 slides with respect to the plate-shaped seal
member 7a. As this is done, the slot in the side plate is sealed
with the seal member 7a, so that the machining fluid never leaks
out through the slot.
[0030] If the servomotor (not shown) for use as Y-axis drive means
is actuated to drive the movable table 1 and the machining tank 2,
moreover, the tank 2 slides in the Y-axis direction with respect to
the lower arm 3. As this is done, the machining tank 2 is sealed
with a seal portion between the lower arm 3 and the seal member
7a.
[0031] The machining fluid is put into the machining tank 2, and
the workpiece is mounted on the workpiece cradle, which is attached
to the movable table 1, and dipped in the machining fluid. Further,
the wire electrode is stretched between the upper wire guide (not
shown) and the lower wire guide on the distal end of the lower arm
3. The wire electrode is run between the upper and lower wire
guides as the movable table 1 is driven in the X-- and Y-axis
directions by X-- and Y-axis drive mechanisms so that the table
moves in any desired direction on an XY-plane (horizontal plane).
At the same time, voltage is applied between the wire electrode and
the workpiece fixed on the movable table 1, whereupon electric
discharge is generated such that the workpiece is subjected to
electric discharge machining.
[0032] A sludge that is formed of a part of the workpiece removed
by the electric discharge and a sludge produced by corrosion of the
wire electrode scatter and float in the machining fluid. If the
sludges get into the seal section 7, a sliding resistance load at
the seal section increases, thereby distorting the lower arm 3. In
consequence, the position of the lower wire guide that is attached
to the distal end of the lower arm 3 fluctuates. Thus, the accuracy
of positioning of the workpiece with respect to the wire electrode
is reduced, so that the machining accuracy is also reduced.
[0033] According to the present embodiment, therefore, the sliding
resistance load at the seal section 7 is detected, and it is
determined whether or not a predetermined reference load is
exceeded by the detected sliding resistance load. If it is
concluded that the reference load is exceeded, an alarm is issued
to prompt maintenance.
[0034] The detection of the sliding resistance load is achieved by
detecting a load acting on the servomotor that moves the movable
table 1 (or the workpiece). The position and velocity of the
servomotor are controlled, and torque (drive current) for
controlling the position or velocity is also controlled. In order
to control the torque, the drive current of the servomotor is
detected by means of a current detector, and the detected current
is fed back for current loop control, whereby the servomotor is
drivingly controlled. The drive current is proportional to the load
on the servomotor as well as to the torque outputted by the
servomotor. Thus, the timing for maintenance is determined by
obtaining the sliding resistance load based on the value of the
detected current (current feedback value) used for the current loop
control.
[0035] Further, it is a well-known technique to provide servo
control means for drivingly controlling the servomotor with a
disturbance estimation observer that estimates the load on the
servomotor by software processing. Accordingly, an increase of the
sliding resistance load can be detected by obtaining the load on
the servomotor by means of the disturbance estimation observer.
[0036] It is when the movable table 1 and the machining tank 2 are
moved in the X-axis direction or the longitudinal direction of the
slot in FIG. 1 that sludges adhere to the seal section 7 to
increase the sliding resistance load, thereby influencing the
positioning accuracy.
[0037] When the machining tank 2 moves with respect to the lower
arm 3 in the axial direction (Y-axis direction) thereof, a sliding
resistance load generated between the lower arm 3 and the seal
member 7a by the movement is a force in the axial direction of the
arm 3, so that the arm 3 is distorted little by the force. It is
necessary, therefore, only that at least the load on the servomotor
4 for driving the X-axis be detected by its drive current or by
means of the disturbance estimation observer.
[0038] FIG. 2 is a schematic block diagram mainly showing a
controller of the wire electric discharge machine according to the
one embodiment of the invention. Reference numeral 10 denotes a
controller that is composed of a numerical controller that controls
the wire electric discharge machine. The controller 10 comprises a
processor 11 and other components including a memory 12, display
unit 13, input means 14 such as a keyboard, interface 15,
individual axis control means 16, input/output circuit 17, etc.,
which are connected to the processor 11 by a bus 18. The memory 12
is formed of a ROM, RAM, etc. Machining programs and the like are
inputted from or outputted to an external storage medium through
the interface 15.
[0039] The individual axis control means 16 controls servomotors
that individually drive the X- and Y-axes for the movement of the
movable table 1, mounted with the machining tank 2, in the X-axis
direction and the Y-axis direction perpendicular thereto, a
servomotor that drives a Z-axis for the movement of the upper wire
guide in a direction perpendicular to both the X-- and Y-axis
directions, and servomotors that individually drive a U-axis and a
V-axis perpendicular thereto for taper machining. Further, the
individual axis control means 16 comprises position and velocity
feedback control means and current feedback control means for those
individual axes. The servomotors for the individual axes are
connected to the individual axis control means 16 through servo
amplifiers 21 for the individual axes. Although not shown in FIG.
2, a position/velocity detector is attached to each servomotor and
configured to feed back a position and a velocity to each axis
control circuit.
[0040] The input/output circuit 17 is connected with a power
circuit 22, which applies voltage for generating electric discharge
between the wire of the wire electric discharge machine and the
workpiece, and a signal lamp 23 for notifying the timing for
maintenance. Further, the input/output circuit 17 is connected with
an automatic wire connection apparatus (not shown) for letting the
wire electrode pass through a wire hole as a workpiece machining
start hole, a wire electrode feeder for feeding the wire electrode,
and other peripheral devices.
[0041] The configuration of the controller of the wire electric
discharge machine shown in FIG. 2 is the same as that of the
controller of a conventional wire electric discharge machine. The
controller of the machine of the present invention utilizes the
hardware of the controller of the conventional machine and is
additionally provided with a new function for the notification of
the maintenance timing.
[0042] FIG. 3 is a flowchart showing maintenance timing detection
notification processing executed by the processor 11 for each
predetermined period during the operation of the wire electric
discharge machine.
[0043] First, a load A that acts on the servomotor 4 for driving
the X-axis is detected (Step S1). For obtaining this load from the
drive current of the motor, the detected drive current value fed
back from the current detector through the axis control means 16 is
read. For estimating the load by means of the disturbance
estimation observer, moreover, disturbance estimation observer
processing is previously incorporated in servo control processing
unit for the servomotor 4, mounted to the X-axis control means 16,
and the load on the servomotor is estimated by using the
disturbance estimation observer.
[0044] The load A obtained in this manner is compared with a
predetermined reference load As (Step S2). If the reference load As
is not exceeded by the obtained load A, this maintenance timing
detection notification processing terminates. If the reference load
As is exceeded by the obtained load A, a warning is issued, a
message suggestive of arrival at the maintenance timing is
displayed on the display unit 13, and the signal lamp 23 is lit by
the input/output circuit 17, whereupon an operator is informed of
the arrival at the maintenance timing (Step S3). The signal lamp 23
may be replaced with an alarm generator that generates an alarm
sound to notify the maintenance timing.
[0045] The reference load As that serves as a criterion for the
identification of the maintenance timing is obtained based on a
feedback signal from the current detector in its best condition or
on the load value estimated by the disturbance estimation observer,
and is previously set and stored in the memory 12.
[0046] When maintenance is tried with a heavy object, such as the
workpiece, on the movable table 1, however, the weight of the
object or workpiece is measured in advance and inputted to the
controller 10, and a correction amount of the reference load As is
calculated from this weight and corrected. Alternatively, a
correspondence table for the correction amount for the weight may
be previously stored in a table format in the memory 12. In this
case, the reference load As can be corrected by obtaining the
correction amount corresponding to the inputted weight.
[0047] Thus, if the workpiece to be machined is mounted on the
movable table 1 by means of the jig with the reference load As
corrected by the weight of the object placed on the movable table
1, the jig need not be removed when the sliding resistance load is
measured. In other words, the sliding resistance load can be
measured to determine the maintenance timing without removing the
jig or the workpiece.
[0048] Further, the load that acts on the servomotor 4 includes
resistances from a guide mechanism and a feed mechanism, such as a
ball screw/nut mechanism, as well as the sliding resistance load at
the seal section 7. The degree of secular variation of the
resistance load can be made much lower than that of the sliding
resistance load at the seal section 7 by periodically supplying a
lubricant by means of an automatic lubricating mechanism or grease
nipples. Therefore, influences of any other parts than the seal
section can be ignored in measuring the sliding resistance load at
the seal section and in comparing the load with the reference load.
After the feed mechanism or the like is maintained with the
lubricant supply or after the seal section 7 is cleaned, moreover,
the servomotor 4 is driven to detect the load (drive current value
or estimated disturbance load), whereby the reference load As is
obtained and set. By doing this, the reference load As can be
updated to meet the machine conditions, so that the maintenance
timing can be automatically appropriately notified as a warning
even if the machine performance varies across the ages. Thus, the
positioning accuracy cannot be reduced, so that reduction of the
machining accuracy can be prevented.
[0049] Instead of detecting the load by detecting the drive current
of the servomotor or by means of the disturbance estimation
observer, moreover, the sliding resistance load at the seal section
may be detected by means of a strain sensor, such as a load cell,
provided in the place where the sliding resistance load at the seal
section is received. In this case, the maintenance timing is
determined depending on whether or not the detected load is heavier
than the reference load.
[0050] In the wire electric discharge machine according to the
embodiment described herein, furthermore, the machining tank is
movable and the lower arm is fixed. However, the present invention
is also applicable to a wire electric discharge machine of a type
such that a machining tank is fixed and a lower arm is movable.
* * * * *